This invention relates generally to golf balls, and, more specifically, to multi-layer golf balls. In particular, this invention relates to a golf ball having a core, at least one intermediate layer, acid at least one cover layer, wherein the intermediate layer comprises an ionomer, or ionomer blend, and further comprises at least one fatty acid or metal salt thereof. The multi-layer golf balls of the present invention provide a higher ball speed and/or a higher coefficient of restitution (“C.O.R.”) than do analogous balls having an identical core and cover layer, but an intermediate layer made from the identical ionomers or ionomer blends but lacking the fatty acid or metal salt.
Until recently, golf balls typically were divided into two general types or groups: 1) two-piece balls, and 2) wound balls (also known as three-piece balls). The difference in the play characteristics of these two types of balls can be quite significant.
Wound balls typically have either a solid rubber, or liquid-filled, center around which many yards of a stretched elastic thread or yarn is wound to form a core. The wound core then is covered with a durable cover material, e.g., an ionomer or other thermoplastic material or a softer cover such as balata or cast polyurethane. Wound balls generally are softer than two-piece balls, and they provide more spin, which enables a skilled golfer to have more control over the ball's flight. In particular, it is desirable for the golfer to be able to impart backspin to the ball, for purposes of controlling its flight and controlling the action of the ball upon landing on the ground. For example, substantial backspin will make the ball stop once it strikes the landing surface instead of bounding forward. The ability to impart backspin onto a golf ball is related to the extent to which the golf ball's cover deforms when it is struck by a golf club. Because conventional wound balls are generally more deformable than are conventional two-piece balls, it is easier to impart spin to wound balls. However, higher spinning wound balls typically travel a shorter distance when struck, as compared to two-piece balls. Moreover, because wound balls generally have a more complex structure, they generally require a longer time to manufacture and are more expensive to produce than are two-piece balls.
Golf balls having a two-piece construction generally are most popular with the recreational golfer, because they are relatively durable and provide maximum distance. Two-piece balls have a single solid core, usually formed of a cross-linked rubber, which is encased by a cover. Typically, the solid core is made of polybutadiene, which is chemically cross-linked with peroxide, or sulfur compounds together with co-cross-linking agent, such as zinc diacrylate. The cover of such balls often comprises tough, cut-proof blends of one or more materials known as ionomers, which typically are ethylene/acrylic acid copolymers or ethylene/acrylic acid/acrylate terpolymers in which some or all of the acid groups are neutralized with metal cations. Such ionomers are commercially available under trademarks such as SURLYN®, which are resins sold commercially by DuPont, of Wilmington, Del., or IOTEK® which is sold commercially by ExxonMobil, of Irving, Tex.
The combination of the above-described core and cover materials provides a “hard” covered ball that is resistant to cutting and other damage caused by striking the ball with a golf club. Further, such a combination imparts a high initial velocity to the ball, which results in increased distance. Due to their hardness, however, these two-piece balls have a relatively low spin rate, which makes them difficult to control, particularly on relatively short approach shots. As such, these balls generally are considered to be “distance” balls. Because the materials of two-piece balls are very rigid, the balls typically have a hard “feel” when struck by a club. Softer cover materials, e.g., balata or softer ionomers or polyurethanes in some instances, have been employed in two-piece balls in order to provide improved “feel” and increased spin rates, although sometimes with a reduction the ball's speed or C.O.R.
The C.O.R. of a one-piece golf ball is a function of it's composition. In two-piece golf balls and multi-layered golf balls, the C.O.R is a function of the various properties of the core, the cover, and any additional layer. Although the United States Golf Association (U.S.G.A.) has not promulgated any limitations on the C.O.R. values for golf balls, it has instituted a rule prohibiting the competitive use in any U.S.G.A.-sanctioned event of a golf ball that can achieve an initial velocity greater than 76.2 meters per second (m/s), or 250 ft/s, when struck by a golf club driver having a velocity of 39.6 m/s, i.e., 130 ft/s (referred to (hereinafter as “the U.S.G.A. test”). However, an allowed tolerance of two percent permits manufacturers to produce golf balls that achieve an initial velocity of 77.7 m/s (255 ft/s).
Regardless of the form of the golf ball, players generally seek a ball that delivers maximum distance, which requires a high initial velocity upon impact. Therefore, in an effort to meet the demands of the marketplace, golf ball manufacturers strive to produce balls delivering initial velocities in the U.S.G.A. test that approximate the U.S.G.A. maximum of 77.7 m/s, or 255 ft/s, as closely as possible. Golf ball manufacturers also generally strive to maximize the ball's C.O.R. without violating the velocity limitation. Also, to maximize distance, it is advantageous if the balls have a lower driver spin rate. Finally it is highly desirable if, while providing increased velocity and distance, the balls also will exhibit a soft shot feel.
Recently, several golf ball manufacturers have introduced multi-layer balls, i.e., balls having at least a core, an intermediate layer or mantle, and one or more cover layers. The goal of these manufacturers has been to overcome some of the undesirable aspects of conventional two-piece balls, e.g., their hard feel. Such a multi-layer structure allows the introduction of new materials of varying hardness, whereby deficiencies in a property in one layer can be mitigated by the introduction of a different material in another layer. For example, to optimize ball hardness and “feel,” blends of copolymeric high-acid ionomers with softer terpolymeric ionomers have been used as a layer material in a golf ball but again, often with a concurrent loss of C.O.R. and/or speed.
Numerous examples of multi-layer combinations are available. For example, U.S. Pat. No. 4,431,193 discloses a golf ball having a multi-layer cover, in which the inner cover layer is a relatively hard, high flexural modulus ionomer resin and the outer cover layer is a relatively soft, low flexural modulus ionomer resin.
Also, U.S. Pat. No. 6,368,237 discloses a multi-layer golf ball comprising a core, an inner cover layer, and an outer cover layer. The inner cover layer comprises a high-acid ionomer or ionomer blend. The outer cover layer comprises a soft, very low-modulus ionomer or ionomer blend, or a non-ionomeric thermoplastic elastomer such as polyurethane, polyester, or polyesteramide. The resulting multi-layer golf ball is said to provide an enhanced distance without sacrificing playability or durability when compared to known multi-layer golf balls.
U.S. Pat. Nos. 6,416,424, 6,416,424, and 6,419,594, likewise, disclose multi-layer golf balls comprising a core, an inner cover layer, and an outer cover layer. The inner cover layer comprises a low-acid ionomer blend. The outer cover layer comprises a soft, very low modulus ionomer or ionomer blend, or a non-ionomeric thermoplastic elastomer such as polyurethane, polyester, or polyesteramide. The resulting multi-layer golf ball is said to provide an enhanced distance without sacrificing playability or durability when compared to known multi-layer golf balls.
U.S. Pat. Nos. 6,503,156 and 6,506,130, likewise, disclose multi-layer golf balls comprising a core, an inner cover layer, and an outer cover layer. The inner cover layer comprises a low-acid ionomer blend. The outer cover layer comprises a soft, non-ionomeric thermoplastic or thermosetting elastomer such as polyurethane, polyester, or polyesteramide. The resulting multi-layered golf ball is said to provide an enhanced distance without sacrificing playability or durability when compared to known multi-layer golf balls.
Another approach to optimizing golf ball performance has been to incorporate selected additives into the polymer compositions used to make the various ball layers, in order to modify the polymer properties. Such additives include the metal salts of various fatty acids. For example, U.S. Pat. Nos. 5,312,857 and 5,306,760 disclose cover compositions for golf ball construction comprising mixtures of ionomer resins and 25-100 parts by weight of various fatty acid salts (i.e., metal stearates, metal oleates, metal palmitates, metal pelargonates, metal laurates, etc.). However, the patents fail to disclose any major effects on ball properties, and fail to disclose that the compositions are useful for parts of a golf ball other than the cover.
Recent attempts to extend the concept of the use of multi-layer covers to mitigate the harsh feel of the harder ionomer materials have also resulted in the development of modified ionomers for use in golf ball compositions. For instance, U.S. Pat. No. 6,100,321 and U.S. Patent Publication No. 2003/0158312 A1 disclose ionomer compositions that are modified with 25 to 100 parts by weight of a fatty acid salt such as a metal stearate, for producing golf balls having good resilience and high softness. Unlike the earlier-mentioned patents, which have employed metal stearates as a filler material, U.S. Pat. No. 6,100,321 and U.S. Patent Publication No. 2003/0158312 A1 contemplates the use of relatively low levels of a stearic acid moiety, particularly metal stearates, to modify ionomers to produce improved resilience for a given level of hardness or PGA Compression values. The stearate-modified ionomers are taught as being especially useful when the ionomer is formulated for use as a golf ball core or center, as a one-piece golf ball, or as a soft golf ball cover. However, there is no disclosure of any ball construction parameters required to produce specific performance properties such as driver velocity or driver spin for three-piece balls.
Subsequent patents have furthered the use of such modified ionomers in golf ball covers. For example, U.S. Pat. No. 6,329,458 discloses a golf ball cover comprising an ionomer resin and a metal “soap,” e.g., calcium stearate. Finally, U.S. Pat. No. 6,616,552 discloses a golf ball including a multi-layer cover, one layer of which includes a heated mixture of an ionomer resin and a metal salt of a fatty acid, e.g., calcium stearate.
It should be appreciated from the foregoing description that there remains a need for a golf ball that can provide maximum C.O.R. without violating the velocity limitation. Also, to maximize distance, it is desirable for such balls to have a lower driver spin rate and to exhibit a soft shot feel. The present invention satisfies this need.